Gas hydrate is a special type of inclusion compound which forms when light hydrocarbon (C.sub.1 -C.sub.4) constituents and other light gases (CO.sub.2, H.sub.2 S, N.sub.2, etc.) physically react with water at elevated pressures and low temperatures. Natural gas hydrates are solid materials, and they do not flow readily in concentrated slurries or solid forms. They have been considered as an industrial nuisance for almost sixty years due to their troublesome properties of flow channel blockage in oil and gas production and transmission systems. In order to reduce the cost of gas production and transmission, the nuisance aspects of gas hydrates have motivated years of hydrate inhibition research supported by the oil and gas industry. (Handbook of Natural Gas, D. Katz etc., pp. 189-221, McGraw-Hill, New York, 1959; Clathrate Hydrates of Natural Gases, E. D. Sloan Jr., Marcel Dekker, Inc. 1991). The naturally occurring natural gas hydrates are also of interest as an alternative energy resource for the industry. (International Conferences on Natural Gas Hydrates, Editors, E. D. Sloan, Jr., J. Happel, M. A. Hnatow, 1994, pp. 225-231--Overview: Gas Hydrates Geology and Geography, R. D. Malone; pp. 232-246--Natural Gas Hydrate Occurrence and Issues, K. A. Kvenvolden).
Natural gas hydrates contain as much as 180 standard cubic feet of gas per cubic foot of solid natural gas hydrates, and several researchers have suggested that hydrates can be used to store and transport natural gases. (B. Miller and E. R. Strong, Am. Gas. Asso. Mon 28(2), 63-1946). The high concentration of gas in the hydrates has led researchers to consider intentionally forming these materials for the purpose of storing and transporting natural gases more safely and cost effectively. U.S. Pat. No. 5,536,893 to Gudmundson discloses a multi-stage process for producing natural gas hydrates. See also Gudmundsson, et al., "Transport of Natural Gas as Frozen Hydrate", ISOPE Conf. Proc., V1, The Hague, NL, June, 1995; "Storing Natural Gas as Frozen Hydrate", SPE Production & Facilities, Feb. 1994.
U.S. Pat. No. 3,514,274 to Cahn et al. teaches a process in which the solid hydrate phase is generated in one or a series of process steps and then conveyed to either storage or directly to a marine transport vessel. This process requires conveyance of a concentrated hydrate slurry to storage and marine transport. Pneumatic conveyance of compressed hydrate blocks and cylinders through ducts and pipelines has also been proposed. See Smirnov, L. F., "New Technologies Using Gas Hydrates", Teor. Osn. Khim. Tekhnol., V23(6), pp. 808-22 (1989), application WO 93/01153, Jan. 21, 1993.
Based upon the published literature (E. D. Sloan, 1991 Clathrate Hydrates of Natural Gases, Marcel Dekker), transporting a concentrated gas hydrate slurry in a pipe from a stirred-tank vessel would appear to be incompatible with reliable operation, or even semi-continuous operation. The blockage of pipes and fouling of the reactors and mixing units are the critical issues. Improved chemical and/or mechanical methods to prevent gas hydrate blockage and fouling remain the focus of current gas hydrate research. (Long, J. "Gas Hydrate Formation Mechanism and Kinetic Inhibition", PhD dissertation, 1994, Colorado School of Mines, Golden, Colo.; Sloan, E. D., "The State-of-the-Art of Hydrates as Related to the Natural Gas Industry,", Topical Report GRI 91/0302, June, 1992; Englezos, P., "Clathrate Hydrates", Ind. Eng. Chem. Res., V32, pp 1251-1274, 1993).
Gas hydrates are special inclusion compounds having a crystalline structure known as clathrate. Gas molecules are physically entrapped or engaged in an expanded water lattice network comprising hydrogen-bonded water molecules. The structure is stable due to weak van der Waals' forces between gas and water molecules and hydrogen-bonding between water molecules within the cage structures. A unit crystal of structure I clathrate hydrates comprises two tetrakaidecahedron cavities and six dodecahedron cavities for every 46 water molecules, and the entrapped gases may consist of methane, ethane, carbon dioxide, and hydrogen sulfide. The unit crystal of structure II clathrate hydrates, on the other hand, contains 8 large hexakaidecahedron cavities and 16 dodecahedron cavities for every 136 water molecules.